This invention relates to an apparatus and method for controlling a turning operation and, more particularly, to a turning control apparatus and method wherein spindle speed may be varied without either defacing screw threads or diminishing the dimensional accuracy of a screw.
A turning machining operation, such as may be applied to the cutting of screws and gears or to the grinding of gear wheels, requires that movement of the workpiece or tool cutting the workpiece be synchronized with the rotation of the machine tool spindle. When cutting a screw, for example, failure to synchronize movement of the cutting tool along the Z-axis with rotation of the spindle results in a screw of poor dimensional precision or causes the formation of double threads or the defacement of threads when the screw is subjected to finishing work. For this reason, a signal is generated at each revolution of the spindle, movement of the cutting tool along the Z-axis is started in synchronization with the signal, and subsequent feed along said axis is made synchronous with spindle rotation, based on the generated signal. Also, in view of a delay attributed to the associated servo system, in the prior art the same rotational speed is adopted for both rough machining and finishing machining. The reason for the latter will now be set forth.
Theoretically, cutting speed can be raised for finishing work because the load sustained during finishing is lighter than that which prevails during rough machining. When the rotational frequency (r.p.m.) of the spindle and the synchronized feed speed along the Z axis are made m times greater than what would prevail during rough machining, however, the amount of delay imposed by the Z-axis servo system differs from the delay which occurs during rough machining. The servo delay d is proportional to the feed speed f and is expressed by the following: EQU d=f/k (1)
where k the servo system gain. The difference in delay between the rough and finishing machining operations would give rise to a screw cutting error, making it impossible to cut a screw to a high precision. It is for this reason that the spindle is rotated at the same speed for both rough machining and finishing.
A disadvantage encountered in the prior art, therefore, is that a highly efficient operation cannot be achieved since it is necessary to perform the finishing process at the slow speed decided by rough machining.